The invention relates to sampling, or commutating, filter-detectors, more particularly to sampling filter-detectors utilizing capacitive storage, or sampling, means having an improved detecting means for the desired tone signal.
In mobile two-way radio communicating systems, for example, when one mobile energizes its transmitter by depressing the push to talk (PTT) switch all other mobile radios whose receivers are on and are tuned to the same carrier frequency will receive the transmission unless the audio portions of the receivers are squelched as is well known in this art. Mobile two-way radio communicating systems include a carrier or noise squelch which function to turn off or quiet the receiver when no carrier, and only noise, is being received. In addition to, or alternatively, such mobile two-way radio communicating systems include selective squelch circuitry which responds to some particular frequency. In the latter systems only those receivers adapted to respond to that particular (selected) frequency will unsquelch, or turn on audio, when the transmitting mobile depresses the PTT switch to transmit that frequency.
The circuit at the mobile receiver for detecting its particular frequency may be of the sampling filter-detector type according to the invention. Moreover the band pass of each filter should be relatively narrow so that there is small or minimal response at frequencies only slightly removed from the tuned frequency.
Sampling filter-detectors utilizing storage capacitors are known. However, these have been relatively complex and expensive in requiring band pass filters following the storage capacitors and post detection filtering all of which require the use of a substantial number of addition components such as capacitors. Moreover, the resultant detected signal was not of the best wave shape.